Field of the Invention
The present invention relates to an antenna unit, a radar system, and a composite sensor device, for monitoring a traveling direction of a vehicle, and is particularly suitable for an antenna unit, a radar device, or a composite sensor device, that uses millimeter-wave radar and is mounted in a passenger compartment.
Description of Related Art
A radar device mounted in a vehicle is used to enhance the safety in traveling of the vehicle (for example, automobile). In particular, it is well known that a vehicle-mounted radar device using the millimeter-wave radar has been highly improved in its function and performance. Hereinafter, the vehicle-mounted radar device using the millimeter-wave radar is referred to as a millimeter-wave radar device in some descriptions.
Conventionally, the typical millimeter-wave radar device is mounted in front of the engine compartment of an automobile. Therefore, when the depth is assumed in the direction perpendicular to the radar aperture plane, the millimeter-wave radar device having the depth length as short as possible is preferable. To meet this requirement, the DBF (Digital Beam Forming) architecture is often employed in the millimeter-wave radar device. The DBF architecture satisfies this requirement, because the DBF architecture can perform scanning at high speed and high precision without a drive for a rotation.
In order to meet the above requirement, the millimeter-wave radar device is also formed in a thin structure and shape by overlapping and bonding a flat plate type of antenna on an electrical substrate. Among those flat plate types of antennas, an antenna of slotted waveguide array architecture has features such as low loss, high gain, and almost no performance variations caused by heat (for example, refer to Japan laid-open patent application No. 2013-032979A, hereinafter refer to as “Patent Document 1”).
Other than the above antenna type, a printed antenna is sometimes used for a thinner type, but practically lower in efficiency than the waveguide slot array architecture due to a large loss in a conductive line (for example, refer to “Millimeter-wave microstrip array antenna for automotive radar systems by Toshiaki Watanabe and three others, IEICE General Conference 2000, B-1-135, p. 135; hereinafter refer to as “Non-Patent Document 1”).
In other aspects, the millimeter-wave radar has been advanced in various technical developments for the purpose of further spread thereof. For example, consideration has been started of mounting the millimeter-wave radar device in a passenger compartment. For a device mounted in the passenger compartment, for example, the International Patent Publication No. WO2006/035510 proposes a structure shown in FIGS. 9 and 10 thereof. This increases the possibility to mount the millimeter-wave radar device even in a car such as a compact car having little space for mounting a radar device in the front of the engine compartment, (i.e., increasing in the number of models accommodating the millimeter-wave radar device).
Further, when the millimeter-wave radar device is mounted in the front of the engine compartment, the vehicle needs to be provided with a protective cover that transmits radio waves, and the millimeter-wave radar device also needs to be structured to be airtight by a device housing and a radome in order to protect the millimeter-wave radar device. On the other hand, when the millimeter-wave radar device is mounted in the passenger compartment, the protective cover, the airtight structure of the millimeter-wave radar device, and the radome is unnecessary. Therefore, it results in the effectiveness which can reduce the cost of the millimeter-wave radar device (i.e., reduction in price).
As the location for mounting the millimeter-wave radar device in the passenger compartment, it is considered appropriate to accommodate the millimeter-wave radar device in a gap between the windshield and the rearview mirror so as not to obstruct the driver's view. This space is also advantageous because the depth of the space is relatively large enough to accommodate the millimeter-wave radar device by placing the substrate approximately horizontally. However, the height of the millimeter-wave radar device is limited to a range of the height in which the millimeter-wave radar device is concealed in the vertical direction of the rearview mirror because of the above reasons.